1. Field of the Invention
This invention relates to a control technique of a communication network.
2. Description of the Related Art
Various wireless communication systems are under developing and are gradually going to become common. For example, as a relatively short-ranged wireless communication system, a wireless LAN (Local Area Network) system is generally widespread.
On the other hand, as a more short-ranged (e.g., 10 m or less) wireless communication system, a WPAN (Wireless Personal Area Network) system is being developed. That can connect consumer apparatus (e.g., computer peripheral apparatus, digital camera, digital video camera or the like) with a printer or a cellular phone.
At present, for the WPAN, a standard is being drawn as a group of IEEE802.15 standards. Specifications for a network topology configuration or a media access protocol are defined in IEEE802.15.3 standard.
Compared with a wireless LAN, the WPAN may be characterized in that each of wireless communication devices dynamically (autonomously) forms a network according to surrounding circumstances or an executing condition of an application. For example, in an infrastructure mode of the wireless LAN, a predetermined wireless communication device called an access point controls each terminal device. To the contrary, in a wireless network in the WPAN (hereinafter called a piconet), a control station called a PNC (Piconet Coordinator) controls a media access of each terminal device (child station), but a predetermined wireless communication device does not necessarily become a PNC. The WPAN wireless communication device is classified as a device with a PNC function and a device without a PNC function. Any device with a PNC function actually generates a piconet and controls it as a PNC.
FIG. 14 is a sequence diagram representing a procedure of generating a piconet in the WPAN. Each of DEV1 and DEV2 in the figure is a WPAN wireless communication device with a PNC function. DEV1 which is powered on (S1401) scans available frequency channels in order (S1402). That is for determining whether a WPAN piconet has been present or not. A PNC of a piconet periodically transmits a beacon frame. Thus, if DEV1 can detect the beacon frame, it can determine that a piconet is present. If a piconet is present, DEV1 will participate in the piconet.
If a piconet is not present at this moment, DEV1 with a PNC function determines that it needs to be a PNC. DEV1 selects an optimum channel from available frequency channels (S1403). Then, DEV1 generates a new piconet by starting periodic transmission of beacon frame in the selected channel (S1404).
When DEV2 is powered on next to DEV1 (S1411), DEV2 scans frequency channels for a certain time period in the same manner as DEV1 (S1412). DEV2 detects a operating piconet of DEV1 as DEV2 receives a beacon frame from DEV1 (S1413). DEV2 transmits a request for participation command to DEV1, which is a control station of a piconet (S1414). In response to the request for participation, DEV1 returns an admission of participation command to DEV2 (S1415). The admission of participation command includes a device identifier newly allocated to DEV2 by DEV1.
With operation above, one of a plurality of wireless communication devices autonomously becomes a control station and creates a WPAN piconet. Thereafter, when a new wireless communication device is powered on, it participates in a piconet in the same procedure as DEV2.
The TDMA (Time Division Multiple Access) system is adopted as a media access method in a WPAN. Generally, in the TDMA system, a terminal device which needs to reserve a data channel requests a control station to allocate a communication band in advance.
FIG. 15 is a diagram showing a principle of the TDMA system in a WPAN. The abscissa indicates time. A superframe 1501 starts with a beacon frame 1502 that is periodically transmitted by a PNC. Each superframe 1501 is divided into time slots 1503 each of which becomes an available time period to be used by each terminal device in wireless communication.
A communication band available for a terminal device is reserved as a time slot on a time axis like this. In each of time slots, only a terminal device which is designated as a transmitting terminal can transmit a wireless frame. A starting time of the time slot and the length of the time slot are indicated in a beacon frame from a control station. In such a time slot, a source device (Source) and a destination terminal device (Destination) are designated.
With the WPAN, a child station (dependent station) participating in a piconet can hierarchically generate a dependent piconet. In such a case, the wireless communication device that generated the dependent piconet behaves as a control station in the dependent piconet. On the other hand, said wireless communication device operates as a child station in a parent piconet. Such a wireless communication device is called a dependent control station or a dependent PNC. As the parent piconet and the dependent piconet are synchronized with each other, wireless frames that are transmitted by different devices never collide with each other in theory.
On the other hand, a plurality of piconets which are independent of each other operate asynchronously in time. Therefore, a wireless signal asynchronously arrives from each piconet at a region in which communication cover-areas of the piconets overlap one another. Accordingly, collision of wireless frames may occur. In an access protocol by the TDMA, such a frame collision degrades data transfer throughput.
As mentioned above, operation of a parent piconet and operation of a dependent piconet are synchronized with each other so that such degradation of throughput can be avoided.
FIG. 16 is a diagram showing a configuration of a parent piconet and a dependent piconet. In FIG. 16, PNC1 is a control station of a parent piconet 1601 and contains a plurality of wireless communication devices. Although DEV2 among the plurality of wireless communication devices becomes a child station in a parent piconet, it has a role of PNC2 as it operates as a control station in a dependent piconet 1602.
FIG. 17 is a diagram showing operation timing for a parent piconet and a dependent piconet. In FIG. 17, a superframe of the parent piconet 1601 is managed by beacon frame 1502 which is periodically transmitted from PNC1. If a time slot which can be used by the dependent piconet 1602 is allocated in the superframe, PNC2, which is a dependent PNC at a starting point of the time slot, transmits beacon frame 1702. A child station in the dependent piconet 1602 operates being synchronized with the beacon frame 1702. Therefore, a wireless frame of the parent piconet 1601 and a wireless frame of the dependent piconet 1602 never collide with each other. Here, a time slot of the dependent piconet 1602 is indicated in the beacon frame 1502 of the parent piconet 1601. Each time slot in a superframe in the dependent piconet 1602 is indicated in the beacon frame 1702 of the dependent piconet 1602.
The Japanese Patent Application Laid-Open 2000-288813 proposes a technique in which a slave station that was not able to receive synchronized signals from a master station changes from a slave mode to a master mode.
The Japanese Patent Application Laid-Open 2004-146883 proposed a method for a plurality of control stations to receive beacon frames one another and avoid collision of wireless frames by adjusting lengths of superframes one another.
In order to further improve data throughput, a network topology needs to be optimized as much as possible.
When a wireless communication device dynamically and autonomously creates a piconet, it can use a function of generating a piconet and a dependent piconet function mentioned above. But no specific algorithm for constructing an optimum network topology by using the functions has been provided yet.
FIG. 18 is a diagram showing a network configuration that causes frame collision among piconets. In the figure, a first wireless communication device 1801 that is operating as a control station forms a first piconet 1810. The first piconet 1810 contains a second wireless communication device 1802 that is operating as a child station.
When a third wireless communication device 1803 is powered on near the first piconet 1810, it first detects whether a piconet is present or not in the vicinity. The third wireless communication device 1803 can receive a wireless frame transmitted from the second wireless communication device 1802. But as the third wireless communication device 1803 is placed at a distant from the first wireless communication device 1801, i.e., a control station of the first piconet 1810, it cannot receive a beacon frame transmitted from the first wireless communication device 1801.
In this case, as the third wireless communication device 1803 determines that it has no piconet to participate in the vicinity, it becomes a control station itself and generates an independent piconet 1820. As the first piconet 1810 and the independent piconet 1820 operate independently and asynchronously, at a place where cover areas of both of the piconets overlap (the place of the second wireless communication device 1802), frames transmitted from the two piconets may collide with each other. It is needless to say that data throughput is reduced, if frame collision occurs.
Briefly speaking, if a plurality of control stations which cannot recognize the others' presence are present, and if a child station which can communicate with the control stations is present, throughput may be reduced to the child station.
According to the Japanese Patent Application Laid-Open 2000-288813, as a plurality of master stations asynchronously transmit beacon frames in the same manner, the above-mentioned problem cannot be solved.
The Japanese Patent Application Laid-Open 2004-146883 assumes that control stations can communicate with one another, the above-mentioned problem cannot be solved either.
The present invention intends to solve at least one of such problems or the other problems. The other problems can be understood through the entire of the specification.